Answer:
Viruses are prone to clumping together. Many times extensive sequence changes may not make a difference in antibody recognition. While sometimes small changes in HA are sufficient for a large antigenic distance. That is, with few mutations, some Hemagglutinins may be much less recognized by the antibodies.
Explanation:
As the virus replicates in the body of infected people, the immune system captures several pieces of viral protein, the antigenic antibodies, and produces antibodies against them. There are other types of immune responses, too, but they do not cause long-term immunity, so that is beside the point. The two main viral targets of the antibodies are Hemagglutinin and Neuraminidase, since they are the most exposed proteins of the virus. When antibodies associate with them, in addition to signaling macrophages and other types of defense cells that that foreign body (the virus) has to be attacked, they can still prevent the virus from working. An antibody that attacks the Hemagglutinin receptor recognition region prevents it from associating with cells. They are the so-called neutralizing antibodies.
